eval.c: Rename has('neovim') feature to has('nvim')

To follow the convention of 'nvim' for technical references

runtime/doc/nvim_clipboard.txt

@@ -36,9 +36,9 @@ by Nvim):
 This should enable the '+' and '*' registers. As an optional step, set the
 'unnamedclip' option to transparently access clipboard using the unnamed
 register. If you use the same |vimrc| for both Vim and Nvim, make sure you
-only set the option when `has('neovim')` is true:
+only set the option when `has('nvim')` is true:
 >
-    if has('neovim')
+    if has('nvim')
       set unnamedclip
     endif
 <

runtime/doc/nvim_python.txt

@@ -30,7 +30,7 @@ simple step-by-step:
 - Add the following snippet to your `vimrc`, before any python plugins are
   loaded:
 >
-  if has('neovim')
+  if has('nvim')
     runtime! plugin/python_setup.vim
   endif
 <

src/nvim/eval.c

@@ -9842,7 +9842,7 @@ static void f_has(typval_T *argvars, typval_T *rettv)
     "windows",
     "winaltkeys",
     "writebackup",
-    "neovim",
+    "nvim",
     NULL
   };
 

src/nvim/testdir/test86.in

@@ -9,7 +9,7 @@ STARTTEST
 :so small.vim
 :set encoding=latin1
 :set noswapfile
-:if !has('python') || has('neovim') | e! test.ok | wq! test.out | endif
+:if !has('python') || has('nvim') | e! test.ok | wq! test.out | endif
 :lang C
 :fun Test()
 :py import vim